I have a Lowe Big John 16'. I have mushroom anchors on anchor mates but I want to get a rubber coated Navy anchor that I can use in swift water and put a float on the end of the rope to release from my anchor fast if I need to. I have never used a Navy anchor and I want to get one to do the job but I don't want to get one heavier than I need. They come 10# 15# and 20#. What size do you think I need?